Technical Field
The present disclosure refers to a testing head for a testing equipment of electronic devices.
The disclosure refers particularly, but not exclusively, to a testing head with vertical probes not bound for testing electronic devices, in particular integrated on wafers and the description which follows is made with reference to this field of application with the only aim to simplify its description.
Description of the Related Art
As it is well known, a testing head is essentially a device suitable for making an electrical connection of a plurality of contact pads of a microstructure, in particular an electronic device integrated on wafers, with corresponding channels of a testing machine which verifies its function, in particular electrically, or generally for making its testing.
The test made on integrated devices aims in particular to detect and isolate faulty devices, in phase of production yet. Normally, the testing heads are then used for the electric testing of the devices integrated on wafers before cutting and mounting the same into a chip-containment package.
A testing head essentially comprises a plurality of movable contact elements or contact probes retained by at least one pair of plates or guides, being substantially plate-shaped and parallel to each other. Such guides are provided with suitable holes and placed at a certain distance to each other, in order to leave a free zone or air space for the movement and the possible deformation of the contact probes. The pair of guides comprises, in particular, upper and lower guides, both provided with guide holes, inside which the contact probes, normally made of special alloys with good electrical and mechanical properties, axially slide.
Furthermore, the testing head is completed with a containment element of the probes, disposed between the upper and lower guides, normally made of ceramic.
The good connection between the contact probes and the contact pads of the device to be tested is assured by the pressure of the testing head on the device itself, whereas the contact probes being movable inside the guide holes made in the upper and lower guides, are subject at such pressure contact to a bending at the inside of the air space between the two guides and to a sliding inside such guide holes. Testing heads of this type are normally indicated as “testing heads with vertical probes”.
Substantially, such testing heads with vertical probes have an air space in which a bending of the contact probes occurs, such bending being favored by a suitable configuration of the probes themselves or of their guides, as shown schematically in FIG. 1, wherein for simplicity of illustration one and only contact probe has been represented, of a plurality of probes being normally comprised in a testing head.
In particular, in FIG. 1, a testing head 1 is schematically shown, comprising at least one upper guide 3 and one lower guide 4, having respective upper and lower guide holes 5, 6 within which at least one contact probe 10 slides.
The testing head 1 also comprises a containment element 7 of the probes, disposed between the upper and lower guides 3, 4.
The contact probe 10 has at least one end or contact tip 8. With the terms end or tip here and in the following description a terminal portion is meant, which is not necessarily pointed. In particular the contact tip 8 abuts against a contact pad 9 of a device 2 to be tested, so making the mechanical and electrical contact between said device and a testing equipment (not shown), the testing head being an end element thereof.
In some cases the contact probes are bound to the head itself at their upper guide in a fixed manner: reference is made to testing heads with blocked probes.
However, testing heads are frequently used with probes not bound in a fixed manner, but kept interfaced to a so-called board by means of a micro-contact board, called “space transformer”: reference is made to testing heads with non-blocked probes.
In such case, the contact probe has a further contact tip towards a plurality of contact pads of the space transformer. The good electrical contact between probes and space transformer is assured in a similar manner to the contact with the device to be tested by means of the pressure of the probes on the contact pads of the space transformer.
The upper and lower guides 3, 4 are suitably spaced by an air space permitting the deformation of the contact probes 10. Finally, the upper and lower guide holes 5, 6 are so dimensioned as to permit a sliding of the contact probe 10 at their inside.
The shape of the deformation caused by the probes and the needed force to produce such deformation depend on many factors, such as:                the physical features of the alloy making the probes;        the offset between guide holes in the upper guide and respective guide holes in the lower guide; and        the distance among the plates.        
Furthermore, it should be noted that the correct working of a testing head is mainly tied to two parameters: the vertical displacement, or over-travel, of the contact probes and the horizontal displacement, or scrub, of the contact tips of such contact probes.
All these features should be evaluated and calibrated when producing a testing head, whereas the good electrical connection between probes and device should be always guaranteed.
In particular, it is important to assure the scrub of the contact tips which permits to “clean” the surfaces of the contact pads, so as to improve the contact made by the testing head throughout its working life.
The contact tips of the contact probes, in particular those for contacting the contact pads of the devices to be tested, are subject during use to a material accumulation, generally indicated as dirt, which reduces their performances.
It is therefore known to make cleaning operations of the contact tips by means of abrasive tissues.
Obviously such cleaning operations involve the consumption of a portion of the tip end of the probes, and they are therefore limited in number by the length of the tips of the probes. In particular, the subsequent abrasion of the tip portion should be limited to the tapered portion of the probe projecting from the lower guide 4, suitable for making a contact tip.